WORK OF THE ROOT 51 



At definite periods of time mark the height of the rising 

 column by means of narrow strips of gummed paper. If 

 the solution is sufficiently strong, the liquid will rise con- 

 siderably, and it will be interesting to increase the length 

 of the tube and determine how high a column can be raised. 

 This can easily be done by connecting a long piece of glass 

 tubing to the funnel by means of a piece of rubber tubing. 



Consider the conditions of the experiment. In the bulb 

 of the funnel is a dense solution separated from a weak 

 solution by a permeable membrane. Under these con- 

 ditions an exchange takes place, but, as we see from the 

 experiment, a very unequal one. On the one hand, the 

 water passes quickly through the membrane, diluting the 

 sugar solution, whilst, on the other, the sugar solution 

 passes slowly into the water. If this exchange goes on, 

 the two solutions will in time become equal in density. 

 The passage of liquids of different densities through a per- 

 meable membrane which originally separated them is called 

 osmosis. 



Let us now compare the conditions of the roots in the 

 soil with those of their counterpart in this experiment. 

 The root-hairs may be regarded as corresponding to the 

 closed funnels, their contents a dense solution, and their 

 walls permeable membranes. The soil-water will now be 

 the weak solution separated from the dense solution within 

 the root-hair by (1) the outer slimy or mucilaginous layer 

 which readily absorbs the soil-water, (2) the permeable 

 cellulose wall, and (3) a very thin lining layer of the living 

 protoplasm known as the plasmatic membrane, which, 

 unlike the parchment membrane, can determine what sub- 

 stances shall enter or leave the cell. From such a com- 

 parison we are able to obtain some idea as to how water 

 is taken up by the root-hairs and passed on to the inner 

 tissues of the plant. The process, however, is extremely 

 complicated, partly owing to the complex nature of the 



D 2 



